Hemimysis anomala originates from the estuaries of the Black Sea, Sea of Azov and Caspian Sea (Fig. 1). It is present at the base of rivers flowing into these seas and in brackish waters. Since the 2000s, H. anomala has been reported worldwide, following intentional or accidental introductions (Audzijonyte et al. 2008). Paradoxically, H. anomala is recognised as critically endangered in its native habitat (Alexandrov 1999; FAO 2005). This situation illustrates how specific environmental pressures and habitat changes can threaten a species in one context while being considered invasive in another, thereby creating a conservation paradox (Marchetti and Engstrom 2016). In its native range, H. anomala faces severe threats from habitat loss, pollution and ecological disturbances (Korzhov 2021). These disturbances, amplified by industrial and agricultural activities (e.g. hydrocarbons, agricultural runoff), alter essential conditions for its survival, impacting water quality and diminishing natural resources and habitats needed by native crustaceans (Zaitsev et al. 2002; Gogaladze et al. 2021; Korzhov 2021). Furthermore, invasive species and ecological shifts heighten competition for resources, further compromising the viability of native populations (Son et al. 2020; Gogaladze et al. 2021). This duality highlights the complex management requirements for H. anomala, where its conservation is critical in its native habitat, but it poses a significant ecological challenge as an invasive species elsewhere.

Figure 1.

Map showing the distribution and dispersal periods of Hemimysis anomala in Central and Western Europe from the Ponto-Caspian area (Black, Azov and Caspian Seas). The shaded area refers to the presumed native range of H. anomala (Ponto-Caspian). Wide grey arrows depict the main dispersal routes and dashed grey arrows indicate intentional transplantation to the Kaunas water reservoir in Lithuania (adapted from Audzijonyte et al. (2008)). Solid blue lines represent major European rivers capable of supporting the dispersal of H. anomala, while dashed red lines represent canals that have facilitated its spread. The ship pictogram indicates transatlantic transport via ballast water discharge to North America, with the first records of H. anomala in Lake Michigan in 2006, likely resulting from multiple European introduction sources (Audzijonyte et al. 2008). Years indicate first records followed by letters indicate the ISO codes of the corresponding country (see Suppl. material 1: table S1).

In Europe, the expansion of H. anomala and its rate of spread through freshwater ecosystems have been facilitated by human activities since the 1940s (Wittmann 2007). The species was intentionally introduced into large lakes and reservoirs within the former Soviet Union, beginning with the Dniepr Reservoir in Ukraine in the late 1950s and followed by the Kaunas Reservoir in Lithuania in 1960, to serve as fish food (Horecký et al. 2005; Arbačiauskas et al. 2010). These introductions were part of a broader acclimatisation effort conducted between 1955 and 1989 to increase the biomass of commercially important fish species (Karpevich 1975). From the Kaunas Reservoir, H. anomala began its expansion into the Baltic Basin via natural dispersal and anthropogenic corridors, including the Curonian Lagoon and connected waterways (Audzijonyte et al. 2008; Arbačiauskas et al. 2017). Molecular analyses confirmed that the Lithuanian populations played a key role as a secondary source for the species’ expansion into the Baltic Sea (Audzijonyte et al. 2008). Its ability to tolerate brackish environments (Table 1) facilitated its establishment in the Baltic Sea, where the first observation was recorded in 1992 in the coastal waters of south-western Finland (Salemaa and Hietalahti 1993). Subsequent sightings occurred along the Swedish coastline in 1995, in the Gulf of Gdańsk in Poland in 2002 (Janas and Wysocki 2005) and in Estonian waters in 2009 (Kotta and Kotta 2010). The rapid rate of expansion, confirmed retrospectively in Hungary in 1997, observed in 1998 in Austria and later in 2005 in Slovakia, can be attributed to the release of ballast water from shipping vessels (Wittmann et al. 1999; Wittmann 2007; Borza 2008; Borza et al. 2011). During the early 1990s, the intensification of river traffic across Eastern and Central Europe facilitated the spread of H. anomala via the Danube-Black Sea Canal, which connects the Danube River to the Black Sea and its tributaries (Kotta and Kotta 2010). The spread was further enabled by the construction of numerous canals, notably the Main-Danube Canal, which was completed in 1992 and links the Danube to the Rhine Rivers and allows the passage of large vessels from the Black Sea to the North Sea (Fig. 1). The pathway, often referred to as the “southern corridor”, further facilitated the spread of the species (Audzijonyte et al. 2008).

Beyond continental Europe, the first reports were from the English Midlands in the United Kingdom in 2004 (Holdich et al. 2006), followed by sightings in southeast England in 2020 (Andrews et al. 2023). In Ireland, the first reports date back to 2008, in the basin of the River Shannon, from which the species spread to Northern Ireland (Minchin and Holmes 2008; Minchin and Boelens 2010; Gallagher et al. 2015). Unlike in North America, H. anomala has mainly colonised watercourses rather than lake ecosystems in Europe (Verslycke et al. 2000; Janas and Wysocki 2005). The first observation of the crustacean in the Rhine Basin (in Germany and the Netherlands) was made in the Neckar River in 1997 and in the Main River in 1998 (Schleuter and Schleuter 1998; Schleuter et al. 1998; Kelleher et al. 1999). It then continued to spread rapidly in the waterways of north-eastern and southern Germany, as well as in the River Meuse in southern Belgium, with sightings in the Galgenweel, near the Westerschelde Estuary and in the Netherlands (Eggers et al. 1999; Verslycke et al. 2000; Vanden Bossche 2002; Rudolph and Zettler 2003; Stich et al. 2009). The southern corridor has been H. anomala’s most likely route into France, where the first observations were made in the Rhône in 2003 (Daufresne et al. 2007) and in the Rhine in 2005 (Dumont 2006). Hemimysis anomala may have now established throughout the Paris region and northern France, extending as far as the Rhône Delta in the Mediterranean (Wittmann and Ariani 2009; Dumont and Muller 2010). In the large and deep peri-alpine lakes of France, while the species has colonised Lake Bourget and Lake Geneva since 2007 (Golaz and Vainola 2013; Frossard and Fontvieille 2018; Lods-Crozet 2020), the first individuals were only recently discovered in Lake Annecy in early 2024, likely due to this system being disconnected from the Rhône River (SILA, pers. com. 2024). The identification of H. anomala in Lake Stechlin in Germany in 2023 now raises concerns about its spread and potential impact on local biodiversity (Dickey et al. 2024).

Hemimysis anomala was first observed in Lake Ontario in the Great Lakes region of North America in 2006 (Ricciardi 2006; Kipp and Ricciardi 2007; Marty et al. 2010), prompting the establishment of a monitoring network to document and assess its expansion across the continent (Suppl. material 1: fig. S3). The species has since colonised other large lakes (Michigan, Muskegon) via their connecting rivers (Pothoven et al. 2007), as well as the St. Lawrence River near Montreal, Canada (Kestrup and Ricciardi 2008; de Lafontaine et al. 2012). Similar to Europe, the arrival and further invasion of H. anomala were facilitated by ballast water exchanges from multiple European source regions. Genetic analyses confirmed that North American populations originated from both Danubian and Baltic lineages (Audzijonyte et al. 2008; Questel et al. 2012), suggesting repeated introductions through ballast water releases. By 2018, it had been found in all five Laurentian Great Lakes (Suppl. material 1: fig. S3; Evans et al. (2018)).

Although the spread of H. anomala has been mainly facilitated by ballast water, other introduction vectors must be considered in isolated environments like Lake Annecy. Amongst these vectors, recreational activities such as boating, diving and fishing can play a significant role (Martínez-Laiz et al. 2019; Morreale et al. 2023). The passive transport of aquatic organisms via contaminated watercraft and biofouling on boat hulls, especially in the absence of strict cleaning protocols, provides an effective mode of dispersal (Kelly et al. 2013; Mohit et al. 2023). Furthermore, fish stocking and the transport of live fish represent an important anthropogenic vector, as the water used in transport can harbour larvae or juveniles of H. anomala (Zajicek et al. 2009; Saccà 2015; Olden et al. 2021). Strengthening regulations on fish transport and ensuring water treatment or filtration before discharge could help mitigate this risk. Lastly, although natural vectors are less studied, extreme weather events such as flooding and passive dispersal by aquatic birds (especially migratory species) could serve as secondary mechanisms of transport, leading to the accidental introduction of these organisms into new habitats (Andrews et al. 2023). These factors highlight the need for comprehensive management strategies to mitigate the risks of H. anomala introductions.

The ecological impacts of Hemimysis anomala establishment in receiving aquatic ecosystems are predominantly based on its high zooplankton consumption capacity (Borcherding et al. 2006; Ricciardi et al. 2012; Lowery et al. 2023). For instance, in the St. Lawrence River, densities of cladocerans, ostracods, rotifers and predatory invertebrates decreased drastically in late summer, corresponding with the proliferation of Mysidae (Sinclair et al. 2016). Similarly, the abundance of cladocerans declined sharply following the invasion of Lake Honderd in the Netherlands by H. anomala (Ketelaars et al. 1999). This predation pressure can disrupt zooplankton community structure, potentially altering food web dynamics and impacting species dependent on zooplankton as a primary food source (Fig. 2).

Figure 2.

Impacts of H. anomala in aquatic food webs. Hemimysis anomala influences food web dynamics through multiple direct and indirect pathways leading interferences with different trophic levels. Solid arrows represent direct trophic interactions, while dashed arrows shown indirect effects. (1) Hemimysis anomala exerts a direct negative impact on phytoplankton through consumption, reducing its biomass that can trigger indirect negative impacts on zooplankton by lowering the availability resources; (2) Hemimysis anomala exerting a direct negative impact on zooplankton biomass by predation that can lead to a positive indirect effect on phytoplankton biomass by reducing grazing pressure from zooplankton. However, the decline in zooplankton may have negative indirect consequences for omni-planktivorous fishes, which rely on zooplankton as a primary food source; (3) Hemimysis anomala serves as a prey for omni-planktivorous fishes, potentially increasing their biomass (direct positive effect). However, by reducing zooplankton availability, H. anomala may impose an indirect negative effect on these fishes due to resource competition. As a consequence, the net effects of H. anomala on omni-planktivorous fishes remain to be clarified; (4) Due to the uncertainty of the lack of effect of H. anomala on omni-planktivorous fishes, the indirect effects of H. anomala on piscivorous fishes remain unclear. However, H. anomala may contribute to contaminant biomagnification by lengthening the food web ultimately impacting the extent of contamination of higher trophic levels; (5) Hemimysis anomala releases nutrients and dissolved organic carbon (DOC) through excretion and partial fragmentation of organic matter during feeding, which may stimulate primary production. This process may play a role in nutrient cycling and biogeochemical processes in aquatic ecosystems, though further research is also needed here to quantify its extent.

In addition to predation, H. anomala may compete directly with native Mysidae for zooplankton, a competition intensified by H. anomala’s higher feeding rates relative to body mass compared to other Mysidae (Dick et al. 2013; Patonai et al. 2024). This competitive advantage is highlighted by the stronger functional response of H. anomala, where its higher attack rates and shorter handling times enable efficient predation, even in the presence of predators, distinguishing it from native Mysidae (Barrios‐O’Neill et al. 2014; Penk et al. 2018). The species may also compete with other native macroinvertebrates with which it shares a trophic niche. However, H. anomala’s impact on such macroinvertebrates (e.g. gammarids) does not appear significant (Marty et al. 2010; Taraborelli et al. 2012). Further, in the gravel pits of Alsace (France), no significant impact of H. anomala on zooplankton resources or Hydra populations has been demonstrated, despite both species feeding on zooplankton and are present at the same depths (Dumont and Muller 2010).

Beyond trophic interactions, H. anomala exhibits the ability to ingest 30 µm plastic particles at a similar rate to microalgae, with a maximum ingestion of about 750 particles per animal per hour (half-saturated at 5,000 particles per ml) (Lowery et al. 2023). Furthermore, H. anomala was associated with high contaminant concentrations, including methylmercury (MeHg), exceeding that of other littoral invertebrates such as amphipods, dreissenid mussels and zooplankton (Zhang et al. 2012; Brown et al. 2022). This capacity raises concerns about the bioaccumulation of contaminants up the food chain, potentially impacting fish and other higher trophic levels. Due to its position in invaded food webs and its prevalence in some places, such as harbours, for instance, where high levels of organic and inorganic pollutants can be detected, H. anomala may contribute to the bioaccumulation of these pollutants. Furthermore, its presence can potentially elongate the food web by introducing additional trophic levels, thereby increasing the risk and magnitude of contaminant transfer to higher trophic levels. Additionally, its high lipid content not only facilitates the retention of organic contaminants, but also provides an energy-rich resource for consumers, raising questions about whether the overall impacts of H. anomala invasion on food webs are beneficial or detrimental.

Despite the potential for negative impacts, H. anomala may contribute positively to certain ecosystems, especially as a food source for various fish species. Isotopic approaches and visual examination of stomach contents suggest that H. anomala can contribute to the diet of various fish species such as yellow perch (Perca flavescens), European perch (Perca fluviatilis), rock bass (Ambloplites rupestris), lake cisco (Coregonus artedi), white perch (Morone americana), alewife (Alosa pseudoharengus), largemouth bass (Micropterus salmoides) and rainbow smelt (Osmerus mordax) (Lantry et al. 2010; Yuille et al. 2012; Gallagher et al. 2015; Geisthardt et al. 2022). The contribution of H. anomala to fish diets appears positively correlated with its abundance and varied amongst seasons and years. However, it is important to note that, despite the presence of swarms of Hemimysis at the same sites, they are found in low quantities or even absent in the stomach contents of the round goby (Neogobius melanostomus) (Lantry et al. 2010; Fitzsimons et al. 2012; Geisthardt et al. 2022). Moreover, the high lipid concentration of these shrimps enables young perch to reach sexual maturity much more quickly than with a diet based on other prey (Zhuravel 1959; Borcherding et al. 2007). These observations suggest that H. anomala could have a significant impact on food web dynamics and the feeding ecology of fish in environments favourable to the species where it occurs at high abundance (Fig. 2; Geisthardt et al. (2022)).

Finally, H. anomala’s omnivorous feeding habits, combined with its ability to actively swim, enable it to exploit both benthic and pelagic habitats. The ecological versatility may mitigate some ecological disruptions caused by other invasive species and also participate to food-web stability through energetic coupling (Rooney et al. 2006; McMeans et al. 2016). This dual habitat exploitation allows H. anomala to adapt to varying environmental conditions and seasonal shifts in resource availability, which likely explains part of its invasive success. For example, its nycthemeral movements into pelagic waters could reduce the negative ecological impact of the energy sink induced by zebra or quagga mussels (Dreissena polymorpha and D. bugensis) (Yuille et al. 2012; Ives et al. 2013). Some of the nutrients captured by these mussels and released as faeces may be assimilated by H. anomala and made available again to consumers such as fish, particularly in areas with high densities of zebra mussels, such as the littoral zones of large lakes (Brown et al. 2022). The ability to reconnect benthic and pelagic food webs particularly relevant in the Laurentian Great Lakes where the process of benthification has been reported (Hecky et al. 2004).

Beyond direct ecological impacts (i.e. predation and competition), H. anomala may exert indirect influences on native and invaded communities by altering trophic interactions, habitat use and ecosystem processes. Recurrent in situ and direct observations suggest a potential interaction between H. anomala and the signal crayfish (Pacifastacus leniusculus), with the possibility that the crayfish may provide a kind of refuge against perch predation (Jacquet 2023). While this interaction has yet to be confirmed, this association may facilitate the persistence and local proliferation of H. anomala populations in benthic habitats, especially during periods of high predation pressure. In habitats shared with native benthic species, H. anomala may indirectly impact these organisms by competing for limited shelter or changing habitat and food availability (Marty et al. 2010; Penk et al. 2018). The presence of H. anomala may not only affect species distributions, but may significantly influence broader ecosystem dynamics, including sediment bioturbation and nutrient cycling (Covich et al. 1999; Ricciardi et al. 2012). For example, crustaceans like Mysis relicta have been shown to influence oxygen fluxes and sediment biogeochemistry through their bioturbation activity (Lindström and Sandberg-Kilpi 2008). Furthermore, the trophic position of H. anomala in aquatic food webs reveals its dual role as a prey item for planktivorous and omnivorous fishes and as a consumer of zooplankton and phytoplankton, potentially disrupting energy flow and nutrient dynamics (Pérez-Fuentetaja and Wuerstle (2014); Fig. 2). By reducing zooplankton grazing pressure, H. anomala may indirectly promote phytoplankton blooms, altering nutrient release and dissolved organic carbon (DOC) cycling (Ricciardi et al. 2012). However, direct consumption of phytoplankton by H. anomala can exert a negative effect, reducing biomass at the base of the food web (Ketelaars et al. 1999; Marty et al. 2012; Sinclair et al. 2016). As a result, the net effect on nutrient dynamics fluctuates between positive and negative, depending on specific ecological contexts and trophic interactions. Additionally, this alteration of trophic dynamics could affect higher trophic levels, such as piscivorous fishes, through cascading effects of reduced prey availability (Pérez-Fuentetaja and Wuerstle 2014; Brown et al. 2022). These interactions raise questions about the ecological relationships of H. anomala with other organisms and its role in structuring invaded and/or endemic communities.

Regular monitoring in aquatic ecosystems is essential to detect H. anomala during the early stages of establishment, making it possible to monitor the colonisation front of the species to reduce its spread. Advanced monitoring methods, including environmental DNA sampling (eDNA), have proven efficient and rapid for detecting the species (Cangelosi et al. 2024; Melliti et al. 2025). The possibility of leaving light-based traps in place for control purposes (Brown et al. 2017) adds a new dimension to ecological monitoring.

To prevent and effectively manage the spread of invasive species, it is important to disinfect objects frequently in contact with infested waterbodies, such as fishing gear, boats, trailers, sampling and diving equipment and waders. These items can unintentionally transport live specimens or propagules to new ecosystems. Disinfection is necessary because H. anomala can survive on damp surfaces or in residual water, enabling its accidental introduction into non-invaded habitats. Several disinfection methods have been developed and tested to evaluate their effectiveness in reducing the spread of this species. Disinfectant-based aquatic treatments (e.g. Virkon™ Aquatic, Virasure™ Aquatic) and the use of steam have demonstrated 100% mortality of H. anomala specimens, suggesting here their effectiveness in inhibiting the spread of this invasive species (De Stasio et al. 2019; Coughlan et al. 2020). Treatments show that spraying is less effective than immersion (De Stasio et al. 2019). Additionally, treatment with hot water at 45 °C for 15 minutes resulted in 99% mortality in H. anomala, making this protocol a simple, rapid and effective biosecurity method for preventing the spread of this species (Anderson et al. 2015). Finally, drying, although less effective, requires approximately 8 days to achieve significant mortality (Anderson et al. 2015). Exposure to ultraviolet (UV) radiation showed a significant reduction in the survival of H. anomala, which has a very low tolerance to UV, reaching a value of 17.8 kJ/m² (Zeisler 2023). These disinfection methods, including UV exposure after validation, should be incorporated into biosecurity protocols to decontaminate equipment and prevent the spread and impact of H. anomala within aquatic ecosystems.

Public awareness of invasive species policy plays a significant role in preventing accidental spread, especially in areas with recreational fishing or boating. Invasive species policies addressing education, such as decontamination protocols for boats and equipment, can reduce the risk of spread. For instance, implementing rigorous cleaning protocols for fishing and water sports equipment and boats is essential to mitigate the risk of unintentional introductions and dispersal (Coughlan et al. 2020; Mohit et al. 2023). This has been proposed for some very aggressive aquatic species during the last decade, for instance for quagga mussels (Wong and Gerstenberger 2015).

Hemimysis anomala appears to have invaded virtually all types of freshwater ecosystems, including lakes and rivers worldwide, as well as brackish environments such as the Baltic Sea. While this invasion is no longer in doubt, the consequences, along with the ecological and socio-economic risks, remain poorly understood. This underscores the need for effective monitoring and management strategies. Our study demonstrates the ecological role of this Mysidae within food webs, particularly through its interference with native zooplankton and its ability to exploit a wide range of habitats. This plasticity poses a threat to local fauna as it may promote changes in community structure and dynamics.

Further research is necessary to fill the gaps in existing knowledge about H. anomala. These studies should include a detailed analysis of its life cycle parameters in the natural environment, such as reproductive patterns, longevity and population dynamics including swarming events. Notably, gut content analyses reveal that swarming may primarily serve as a feeding function rather than reproduction (Wachala et al. 2025). It is also important to investigate its periods and sites of reproduction, along with its interactions with biotic and abiotic factors, specify the effect of temperature on its reproductive cycle, as well as prey-predator interactions and food resources. More detailed studies of migration and homing behaviour, particularly their energetic demands, physiological drivers such as sensory mechanisms and responses to environmental cues, could also provide valuable insights into the ecophysiology of the animal. As recently observed in H. margalefi, which relies on the chemical signature of its habitat to navigate circadian migrations (Derrien et al. 2024), H. anomala might similarly depend on such mechanisms. Investigating these processes, particularly by identifying and characterising the chemical compounds involved, would improve our understanding of its behavioural ecology. Although H. anomala has established populations in several brackish environments, including the Baltic Sea (Salemaa and Hietalahti 1993; Janas and Wysocki 2005; Kotta and Kotta 2010), almost nothing is known about its long-term persistence and ecological interactions in such habitats. The species’ ability to tolerate variable salinities (Table 1), its reproductive success and its competitive interactions with native and non-native species in estuaries remain insufficiently documented. Additionally, the role of H. anomala in nutrient cycling and dissolved organic carbon (DOC) dynamics remains poorly understood. While this species releases nutrients and DOC through excretion and organic matter decomposition, the extent to which these processes influence primary production and microbial activity remains uncertain. Identifying its impact on nutrient remineralisation, microbial interactions and potential contributions to biogeochemical cycles would refine our understanding of its ecological role in aquatic ecosystems. Finally, its influence on higher trophic levels, particularly piscivorous fishes, remains unclear and requires further investigation. Hemimysis anomala interacts with these predators both directly, through biomagnification and contaminant transfer and, indirectly, via contrasting trophic effects (Fig. 2). However, its net impact on piscivorous fish populations is not well established. Further research is needed to determine how these mechanisms vary across ecosystems and influence food web stability.

We believe that expanding our knowledge of the distribution of H. anomala is another priority. A multi-scale and multi-methods approach combining diving, eDNA and remotely operated vehicle surveys would enable us to map its distribution across and within ecosystems. To refine our understanding of its place in food webs, this will require the application and development of complementary approaches such as underwater video, metabarcoding and immunochemical analyses.

The creation of participatory diving networks can mobilise recreational divers to report the presence of H. anomala, thereby increasing monitoring efforts in often inaccessible areas. These collaborative initiatives not only collect valuable data, but also raise community awareness of the problem of invasive species.

This study demonstrates the potential of citizen science to enhance invasive species monitoring while promoting public engagement in biodiversity conservation. Through participatory diving networks, significant data were gathered on H. anomala’s presence, habitat use and seasonality, showcasing the benefits of collaborative initiatives (Appendix 1). Observations were conducted over an annual cycle, recording H. anomala occurrences and habitat types. Divers contributed data using standardised questionnaires, which included four habitat categories (e.g. benthic sediments covered by quagga mussels, crevices under rocky outcrops, wrecks and artificial shelters and the water column). They also estimated relative abundances of the species and provided additional information, such as life stages (adult vs. juvenile).

By combining scientific diving with citizen science, this study elucidated a clear seasonal pattern in the habitats occupied by H. anomala (Fig. 3). Winter swarms, often exceeding 160,000 individuals in approximately 5 m³ and occasionally occupying volumes up to 100 m³, were widely observed at depths of 10 to 20 metres. In contrast, summer sightings were rare, with individuals sheltering under rocks or at greater depths (approximately 1000 individuals per 5 m³). This seasonal pattern underscores the ecological flexibility of H. anomala and its adaptation to varying environmental pressures. These results demonstrate the value of citizen science in advancing invasive species monitoring, offering both enhanced data collection capabilities and a means of engaging the public in biodiversity conservation efforts.

Figure 3.

Percentages of habitat types occupied by H. anomala observed during the 2020–2021 seasons in Lake Geneva, at the Saint-Disdille pilot site (Thonon-les-Bains, France; Appendix 1). Data were collected through scientific diving and citizen science, with the total number of dives indicated below the bars (n). Observed habitats include the water column (†), anthropogenic structures such as wrecks and concrete pipes (‡), benthic habitats (§) and cavities, caves or rocks (|). The video reveals what winter swarms look like in Lake Geneva (https://hal.science/hal-04820062).